The present invention relates to measurements of a hydrostatic and/or fast-changing pressure by optical means.
A conventional fiber optic pressure sensor comprises a capillary with an optical fiber mounted in the capillary along its axis, and a flexible diaphragm attached to one end of the capillary [G. He, F. W. Cuomo, A. J. Zuckerwar. Diaphragm size and sensitivity for fiber optic pressure sensors. Proc.SPIE, 1991, vol.1584, p.p.152-156]. In the fiber optic sensor, a light signal for measuring the diaphragm deflection is supplied over one transmit multi-mode optical fiber with the core diameter of 50 xcexcm and the cladding diameter of 120 xcexcm, and the light reflected from the pressure-deflected diaphragm is collected by six similar multi-mode optical fibers arranged around the transmit optical fiber and forming an optical fiber bundle with a dense regular packing of optical fibers. A sensitive member of the sensor is a mylar metallized diaphragm 25.4 xcexcm thick, this imparting high-frequency characteristics to the sensor.
A disadvantage of this sensor is that it does not exhibit high sensitivity due to the use of multi-mode optical fibers which prevent formation of an interferometer for measuring pressure effects, the sensor also suffers considerable light losses when collecting the light reflected by the diaphragm in the optical fiber cores which do not embrace closely the light field, and the portion of the output signal light depends crucially on the diaphragm size and the distance between the end of the optical fiber bundle and the diaphragm. The above pressure sensor is essentially the analog instrument with a low diaphragm displacement-to-optical signal conversion conductance.
The most pertinent prior art is a conventional optical fiber pressure sensor comprising a capillary in which a single-mode optical fiber is secured so that to move along the axis of the capillary [K. A. Murphy, M. F. Gunter, A. Wang, R. O. Claus, A. M. Vengsarkar. Extrinsic Fabry-Perot Optical Fiber Sensor. 8th Optical Fiber Sensor Conf., Jan.29-31, 1992, Monterey, Calif. Conf. Proc., p.193-196]. This interferometer sensor is based on a low Q-factor Fabry-Perot interferometer formed by shattered end faces with the 4% Fresnel reflection of a single-mode optical fiber and an end face of a multi-mode optical fiber secured inside a glass capillary with an epoxy. The sensor provides sine wave interferometric response to the pressure changing the gap between the end faces (cavity length), and thereby exhibits a high pressure-to-cavity length (and to sensor output, respectively) conversion conductance.
A disadvantage of the above sensor is that it is impossible to sense a gas or liquid pressure to a high accuracy because the multi-mode optical fiber length acting as a movable mirror is fixed in the structure, and the task of connecting the multi-mode optical fiber to the diaphragm and arrangement of the diaphragm per se has not been solved.
The object of the present invention is to improve the sensitivity and to extend the dynamic range of measurements, as well to enhance the temperature and vibration stability of a pressure sensor.
The above object is attained owing to the fact that in the first variant of a pressure sensor, a movable mirror of the interferometer is a metallic or metallized flexible diaphragm with a diameter which considerably exceeds the external diameter of the optical fiber, this providing the possibility of precision displacement of the diaphragm in the center at a distance which is more than by the order of magnitude greater than the laser radiation wavelength, and extending the dynamic range of pressure measurements, and in the second variant of a pressure sensor, a flexible diaphragm is an organosilicon polymer with a short length of a multi-mode optical fiber pasted-in into the diaphragm, the multi-mode optical fiber length having a flat end face and acting as a movable mirror of the measuring interferometer. Furthermore, proposed is a method for fabricating a flexible diaphragm with a pasted-in optical fiber length for a highly sensitive sensor.
The inventions comprising variants of an apparatus and a method for fabricating a component of the apparatus are united by a single inventive idea.